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<FONT color="green">001</FONT>    /*<a name="line.1"></a>
<FONT color="green">002</FONT>     * Licensed to the Apache Software Foundation (ASF) under one or more<a name="line.2"></a>
<FONT color="green">003</FONT>     * contributor license agreements.  See the NOTICE file distributed with<a name="line.3"></a>
<FONT color="green">004</FONT>     * this work for additional information regarding copyright ownership.<a name="line.4"></a>
<FONT color="green">005</FONT>     * The ASF licenses this file to You under the Apache License, Version 2.0<a name="line.5"></a>
<FONT color="green">006</FONT>     * (the "License"); you may not use this file except in compliance with<a name="line.6"></a>
<FONT color="green">007</FONT>     * the License.  You may obtain a copy of the License at<a name="line.7"></a>
<FONT color="green">008</FONT>     *<a name="line.8"></a>
<FONT color="green">009</FONT>     *      http://www.apache.org/licenses/LICENSE-2.0<a name="line.9"></a>
<FONT color="green">010</FONT>     *<a name="line.10"></a>
<FONT color="green">011</FONT>     * Unless required by applicable law or agreed to in writing, software<a name="line.11"></a>
<FONT color="green">012</FONT>     * distributed under the License is distributed on an "AS IS" BASIS,<a name="line.12"></a>
<FONT color="green">013</FONT>     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.<a name="line.13"></a>
<FONT color="green">014</FONT>     * See the License for the specific language governing permissions and<a name="line.14"></a>
<FONT color="green">015</FONT>     * limitations under the License.<a name="line.15"></a>
<FONT color="green">016</FONT>     */<a name="line.16"></a>
<FONT color="green">017</FONT>    package org.apache.commons.lang3.concurrent;<a name="line.17"></a>
<FONT color="green">018</FONT>    <a name="line.18"></a>
<FONT color="green">019</FONT>    import java.util.concurrent.atomic.AtomicReference;<a name="line.19"></a>
<FONT color="green">020</FONT>    <a name="line.20"></a>
<FONT color="green">021</FONT>    /**<a name="line.21"></a>
<FONT color="green">022</FONT>     * &lt;p&gt;<a name="line.22"></a>
<FONT color="green">023</FONT>     * A specialized {@code ConcurrentInitializer} implementation which is similar<a name="line.23"></a>
<FONT color="green">024</FONT>     * to {@link AtomicInitializer}, but ensures that the {@link #initialize()}<a name="line.24"></a>
<FONT color="green">025</FONT>     * method is called only once.<a name="line.25"></a>
<FONT color="green">026</FONT>     * &lt;/p&gt;<a name="line.26"></a>
<FONT color="green">027</FONT>     * &lt;p&gt;<a name="line.27"></a>
<FONT color="green">028</FONT>     * As {@link AtomicInitializer} this class is based on atomic variables, so it<a name="line.28"></a>
<FONT color="green">029</FONT>     * can create an object under concurrent access without synchronization.<a name="line.29"></a>
<FONT color="green">030</FONT>     * However, it implements an additional check to guarantee that the<a name="line.30"></a>
<FONT color="green">031</FONT>     * {@link #initialize()} method which actually creates the object cannot be<a name="line.31"></a>
<FONT color="green">032</FONT>     * called multiple times.<a name="line.32"></a>
<FONT color="green">033</FONT>     * &lt;/p&gt;<a name="line.33"></a>
<FONT color="green">034</FONT>     * &lt;p&gt;<a name="line.34"></a>
<FONT color="green">035</FONT>     * Because of this additional check this implementation is slightly less<a name="line.35"></a>
<FONT color="green">036</FONT>     * efficient than {@link AtomicInitializer}, but if the object creation in the<a name="line.36"></a>
<FONT color="green">037</FONT>     * {@code initialize()} method is expensive or if multiple invocations of<a name="line.37"></a>
<FONT color="green">038</FONT>     * {@code initialize()} are problematic, it is the better alternative.<a name="line.38"></a>
<FONT color="green">039</FONT>     * &lt;/p&gt;<a name="line.39"></a>
<FONT color="green">040</FONT>     * &lt;p&gt;<a name="line.40"></a>
<FONT color="green">041</FONT>     * From its semantics this class has the same properties as<a name="line.41"></a>
<FONT color="green">042</FONT>     * {@link LazyInitializer}. It is a &amp;quot;save&amp;quot; implementation of the lazy<a name="line.42"></a>
<FONT color="green">043</FONT>     * initializer pattern. Comparing both classes in terms of efficiency is<a name="line.43"></a>
<FONT color="green">044</FONT>     * difficult because which one is faster depends on multiple factors. Because<a name="line.44"></a>
<FONT color="green">045</FONT>     * {@code AtomicSafeInitializer} does not use synchronization at all it probably<a name="line.45"></a>
<FONT color="green">046</FONT>     * outruns {@link LazyInitializer}, at least under low or moderate concurrent<a name="line.46"></a>
<FONT color="green">047</FONT>     * access. Developers should run their own benchmarks on the expected target<a name="line.47"></a>
<FONT color="green">048</FONT>     * platform to decide which implementation is suitable for their specific use<a name="line.48"></a>
<FONT color="green">049</FONT>     * case.<a name="line.49"></a>
<FONT color="green">050</FONT>     * &lt;/p&gt;<a name="line.50"></a>
<FONT color="green">051</FONT>     *<a name="line.51"></a>
<FONT color="green">052</FONT>     * @since 3.0<a name="line.52"></a>
<FONT color="green">053</FONT>     * @version $Id: AtomicSafeInitializer.java 1088899 2011-04-05 05:31:27Z bayard $<a name="line.53"></a>
<FONT color="green">054</FONT>     * @param &lt;T&gt; the type of the object managed by this initializer class<a name="line.54"></a>
<FONT color="green">055</FONT>     */<a name="line.55"></a>
<FONT color="green">056</FONT>    public abstract class AtomicSafeInitializer&lt;T&gt; implements<a name="line.56"></a>
<FONT color="green">057</FONT>            ConcurrentInitializer&lt;T&gt; {<a name="line.57"></a>
<FONT color="green">058</FONT>        /** A guard which ensures that initialize() is called only once. */<a name="line.58"></a>
<FONT color="green">059</FONT>        private final AtomicReference&lt;AtomicSafeInitializer&lt;T&gt;&gt; factory =<a name="line.59"></a>
<FONT color="green">060</FONT>                new AtomicReference&lt;AtomicSafeInitializer&lt;T&gt;&gt;();<a name="line.60"></a>
<FONT color="green">061</FONT>    <a name="line.61"></a>
<FONT color="green">062</FONT>        /** Holds the reference to the managed object. */<a name="line.62"></a>
<FONT color="green">063</FONT>        private final AtomicReference&lt;T&gt; reference = new AtomicReference&lt;T&gt;();<a name="line.63"></a>
<FONT color="green">064</FONT>    <a name="line.64"></a>
<FONT color="green">065</FONT>        /**<a name="line.65"></a>
<FONT color="green">066</FONT>         * Get (and initialize, if not initialized yet) the required object<a name="line.66"></a>
<FONT color="green">067</FONT>         *<a name="line.67"></a>
<FONT color="green">068</FONT>         * @return lazily initialized object<a name="line.68"></a>
<FONT color="green">069</FONT>         * @throws ConcurrentException if the initialization of the object causes an<a name="line.69"></a>
<FONT color="green">070</FONT>         * exception<a name="line.70"></a>
<FONT color="green">071</FONT>         */<a name="line.71"></a>
<FONT color="green">072</FONT>        public final T get() throws ConcurrentException {<a name="line.72"></a>
<FONT color="green">073</FONT>            T result;<a name="line.73"></a>
<FONT color="green">074</FONT>    <a name="line.74"></a>
<FONT color="green">075</FONT>            while ((result = reference.get()) == null) {<a name="line.75"></a>
<FONT color="green">076</FONT>                if (factory.compareAndSet(null, this)) {<a name="line.76"></a>
<FONT color="green">077</FONT>                    reference.set(initialize());<a name="line.77"></a>
<FONT color="green">078</FONT>                }<a name="line.78"></a>
<FONT color="green">079</FONT>            }<a name="line.79"></a>
<FONT color="green">080</FONT>    <a name="line.80"></a>
<FONT color="green">081</FONT>            return result;<a name="line.81"></a>
<FONT color="green">082</FONT>        }<a name="line.82"></a>
<FONT color="green">083</FONT>    <a name="line.83"></a>
<FONT color="green">084</FONT>        /**<a name="line.84"></a>
<FONT color="green">085</FONT>         * Creates and initializes the object managed by this<a name="line.85"></a>
<FONT color="green">086</FONT>         * {@code AtomicInitializer}. This method is called by {@link #get()} when<a name="line.86"></a>
<FONT color="green">087</FONT>         * the managed object is not available yet. An implementation can focus on<a name="line.87"></a>
<FONT color="green">088</FONT>         * the creation of the object. No synchronization is needed, as this is<a name="line.88"></a>
<FONT color="green">089</FONT>         * already handled by {@code get()}. This method is guaranteed to be called<a name="line.89"></a>
<FONT color="green">090</FONT>         * only once.<a name="line.90"></a>
<FONT color="green">091</FONT>         *<a name="line.91"></a>
<FONT color="green">092</FONT>         * @return the managed data object<a name="line.92"></a>
<FONT color="green">093</FONT>         * @throws ConcurrentException if an error occurs during object creation<a name="line.93"></a>
<FONT color="green">094</FONT>         */<a name="line.94"></a>
<FONT color="green">095</FONT>        protected abstract T initialize() throws ConcurrentException;<a name="line.95"></a>
<FONT color="green">096</FONT>    }<a name="line.96"></a>




























































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